The Realms of the Gods — Tamora Pierce

The final battle

Daine and the mage Numair are faced with certain death when they are suddenly swept into the mystical realms of the gods, where Maine learns the secrets of her past. But she and Numair are both needed in the mortal world to help fight the desperate war that is raging in Tortall. And so they undertake the journey home--a dangerous journey that will teach them about life and about each other. Their path leads them to the final clash, which involves both mortals and immortals a battle in which the only chance for Tortall's future lies with Maine and her wild magic.

Set in Tortall during the reign of King Jonathan III and Queen Thayet, The Realms of the Gods is the final book in The Immortals series, which chronicles a time when the world is invaded by immortal creatures.